1. Field of the Invention
The present invention relates to a steam generator, and particularly, to a pressurized fluidized bed boiler including control means capable of performing excellent controls of steam temperature and air-fuel ratio.
2. Description of the Prior Art
D. Bunthoff and others have published "PRESSURIZED FLUIDIZED BED COMBUSTION AND FIRST EXPERIENCE WITH THE BABCOCK 15 MWth PFEBC PILOT PLANT" 1989, INTERNATIONAL CONFERENCE ON FLUIDIZED BED COMBUSTION, Vol.1, 219 pp, which is one example of prior art pressurized fluidized bed boilers including a furnace disposed within a pressure vessel and having a structure in which a lower portion thereof, specifically, an area with an in-bed heat-transfer tube disposed therein, is divided into two zones, and gases are joined to each other at an upper area of the furnace
From the above International Conference Report, a construction is easily conceived which is different from the example in the above International Conference Report and which comprises an evaporator tube 8 and a portion 9a of a superheater tube 9 disposed as heat transfer tubes in one zone 1a of a furnace, and a reheater tube 10 and the other portion of the superheater tube 9 disposed on the other zone 1b of the furnace, as shown in FIG. 3. In the example shown in FIG. 3, the temperature of steam in an outlet of the superheater 9 is controlled by permitting a bed material within the furnace zone 1a to flow into and out of a tank 7a to vary the level of the bed. The temperature of steam in an outlet of the reheater 10 is also controlled by permitting the bed [material within the other furnace zone 1b to flow into and out of a tank 7b to vary the level of the bed. Thus, a reduction in efficiency of the plant is prevented without use of a reheater spray. The reheater spray may be used in an emergency when the temperature of steam in the outlet of the reheater has been abnormally risen.
Combustion exhaust gases resulting from combustion in the furnace zones 1a and 1b are joined to each other at their upper portions and supplied via an exhaust gas duct 13 into a gas turbine 15. Air within an air duct 4 is compressed by an air compressor 16 driven by the gas turbine 15 and supplied into a pressure vessel 2. An exhaust gas discharged from the gas turbine 15 is cooled in an exhaust gas cooler 17 and then released through a stack 18 into the atmosphere.
In the example shown in FIG. 3, the bed levels of the furnace zones 1a and 1b are governed by the temperatures of steams in the outlets of the superheater 9 and the reheater 10, and hence, they are necessarily not equal to each other and are largely different from each other depending upon operational conditions of the plant such as the stopping and the starting of the plant or a variation in load of the plant. Therefore, in order to maintain a proper fluidizing speed and a proper air-fuel ratio required to maintain a good combustion in both the furnace zones 1a and 1b, an adjusting valve for constantly monitoring and adjusting the flow rate of air supplied into the furnace zones 1a and 1b is required. In this case, however, as shown in FIG. 3, air flow rate adjusting valves 3a and 3b in combustion air ducts 4a and 4b must be placed in the pressure vessel 2, resulting in a problem arisen in the reliability and maintenance of the air flow rate adjusting valves 3a and 3b.
Further, it is impossible to independently measure the oxygen concentrations in outlets of fluidized beds in the furnace zones 1a and 1b, resulting in a problem arisen even in the control of the air-fuel ratio.
Thereupon, an example is shown in FIG. 4, in which the entire furnace arrangement is divided in order to solve the above problems (see Japanese Patent Application Laid-open No. 156201/91). In the example shown in FIG. 4, the furnace arrangement 1 accommodated in a single pressure vessel 2 is completely divided into two furnaces 1a and 1b. This ensures that the oxygen concentration in exhaust gas outlets of the furnaces 1a and 1b can be monitored, thereby enabling a reliable control of the air-fuel ratio. In the prior art technique shown in FIG. 4, however, in the respect of independent control of the flow rates of air in the furnaces 1a and 1b, an approach is not taken into consideration, thereby leaving the same problem as in the prior art technique shown in FIG. 3. Of course, it is possible to independently control the flow rates for each of the furnaces 1a and 1b, but for the same reason as in the example shown in FIG. 3, the problem is left in the respect of the reliability of maintenance of the air flow rate adjusting valves.
It is also conceived to place air tubes 4a and 4b to extend from the outside of a pressure vessel 2 directly into divided furnace portions 1a and 1b within a pressure vessel 2. When this construction is employed, a compressed air source is additionally required for pressurizing the interior of the pressure vessel 2. An air tube system 19 may be mounted for the compressed air source to extend from an air compressor 16 and measurement-control attachments (a valve 20 and the like) may be mounted in the air tube system 19, thereby bringing about a complication in entire plant and an increase in cost. In addition, after the interior of the pressure vessel 2 has been once pressurized the compressed air used therefor is not required and hence, the valve 20 is closed to stop the supply of the air. Consequently, the air in the pressure vessel 2 may be stagnated and subjected to a radiant heat from the furnaces 1a and 1b, so that the temperature of the atmosphere within the pressure vessel 2 is gradually risen. Therefore, the design temperatures of the pressure vessel 2 and the structures within the pressure vessel 2 must be risen and thus, it is required to increase the all thickness of a material or to increase the grade of the material, thereby bringing about an increase in cost.